1. Field of the Invention
This invention relates to the field of oxides for integrated circuits, particularly metal-oxide-semiconductor (MOS) circuits.
2. Prior Art
In the fabrication of metal-oxide-semiconductor (MOS) circuits, "front end" processing is used to form thick field oxide regions at predetermined sites on the substrate. These thick field oxide regions are used primarily for isolation, for example, to prevent parasitic conduction which would otherwise occur when conductive lines cross these regions.
The most common process for forming these field oxide regions employs a silicon nitride layer. Openings are formed in this layer at the predetermined sites and then an oxide is grown on the substrate. The oxide grows primarily at the openings and not at the regions protected by the silicon nitride layer. As the oxide grows, its lifts the edges of the silicon nitride layer at the openings. In these areas of lifted silicon nitride, the field oxide becomes thinner, and is generally tapered. These tapered regions, which occur at the nitride-substrate interface, are sometimes referred to as "bird beaks" since their cross section resembles a bird's beak.
The bird beak oxide regions, as will be described in more detail in conjunction with FIG. 1, consume substrate area without providing benefits. For example, in the beak region of the oxide is generally too thin to provide good field isolation and yet too thick to be used as part of active devices.
Several solutions have been proposed to reduce bird's beak or otherwise save substrate area. U.S. Pat. No. 4,352,236 teaches the formation of two oxide fields, one thicker than the other. The thinner oxide has a smaller beak and is used where less isolation is required.
The control of bird's beaks was addressed by J. Hui, et al, in "Selective Oxidation Technologies for High Density MOS", IEEE Electronic Devices Letters, Vol. EDL-2, No. 10, October, 1981 p. 244, and in "Electrical Properties of MOS Devices Made with SILO Technology", IEDM, 1982, p. 220. Hui teaches the forming of a nitride layer directly on the substrate, with no intervening oxide layer.
The forming of local oxide regions using silicon nitride is discussed in U.S. Pat. Nos. 3,751,722 and 3,752,711. Neither of the patents addresses the problems of bird's beaks.